Contrib Mineral Petrol (2000) 138: 249±264 Ó Springer-Verlag 2000 Peter W. Reiners á Paul E. Hammond Juliet M. McKenna á Robert A. Duncan Young basalts of the central Washington Cascades, ¯ux melting of the mantle, and trace element signatures of primary arc magmas Received: 2 March 1999 / Accepted: 18 August 1999 Abstract Basaltic lavas from the Three Sisters and correlated with the amount of SC added to it. Distinctive Dalles Lakes were erupted from two isolated vents in the CAB and OIB-like trace element compositions are best central Washington Cascades at 370±400 ka and explained by a ¯ux-melting model in which dF/dSC 2.2 Ma, respectively, and have distinct trace element decreases with increasing F, consistent with isenthalpic compositions that exemplify an important and poorly (heat-balanced) melting. In the context of this model, understood feature of arc basalts. The Three Sisters la- CAB trace element signatures simply re¯ect large de- vas are calc-alkaline basalts (CAB) with trace element grees of melting of strongly SC-¯uxed peridotite along compositions typical of most arc magmas: high ratios of relatively low dF/dSC melting trends, consistent with large-ion-lithophile to high-®eld-strength elements derivation from relatively cold mantle. Under other (LILE/HFSE), and strong negative Nb and Ta anoma- conditions (i.e., small degrees of melting or large degrees lies. In contrast, the Dalles Lakes lavas have relatively of melting of weakly SC-¯uxed peridotite [high dF/ low LILE/HFSE and no Nb or Ta anomalies, similar to dSC]), either OIB- or MORB (mid-ocean ridge basalt)- ocean-island basalts (OIB). Nearly all Washington like compositions are produced. Trace element and iso- Cascade basalts with high to moderate incompatible topic compositions of Washington Cascade basalts are element concentrations show this CAB or OIB-like easily modeled by a correlation between SC and F across compositional distinction, and there is pronounced a range of mantle temperatures. This implies that the divergence between the two magma types with a large dominant cause of arc magmatism in this region is ¯ux compositional gap between them. We show that this melting of the mantle wedge. trace element distinction can be easily explained by a simple model of ¯ux-melting of the mantle wedge by a ¯uid-rich subduction component (SC), in which the degree of melting (F) of the peridotite source is Introduction Arc basalts are typically enriched in LILE and other ¯uid P.W. Reiners (&)1 mobile elements (e.g., Cs, Rb, Ba, Sr, K, U, Th, LREE) Division of Geological and Planetary Sciences, relative to most HFSE (especially Nb and Ta, and to MS 100-23, California Institute of Technology, Pasadena, CA 91125, USA lesser degrees Hf, Zr, and Ti). This characteristic is gen- e-mail: [email protected] erally thought to be caused by addition of a LILE-rich ¯uid or melt derived from a subducted slab to a mantle P.E. Hammond Department of Geology, Portland State University, wedge source (e.g., Gill 1981; Tatsumi et al. 1986; Portland, OR 97207, USA Hawkesworth et al. 1993, 1994; Stolper and Newman J.M. McKenna 1994). Basalts displaying such trace element features are Montgomery Watson Americas, Inc., Pasadena, CA 91101, USA commonly termed CAB (e.g., Leeman et al. 1990; Conrey R.A. Duncan et al. 1997; Bacon et al. 1997). Many arc basalts, how- College of Oceanic and Atmospheric Sciences, ever, especially in the Cascades, have relatively high in- Oregon State University, Corvallis, compatible element concentrations but do not have high OR 97331, USA LILE/HFSE or negative Nb and Ta anomalies. Because Present address: these trace element characteristics are common to many 1 Department of Geology, Washington State University, Pullman, ocean-island basalts, these have been termed intraplate-, WA 99164, USA or OIB-like basalts (e.g., Luhr et al. 1989, Luhr 1997; Editorial responsibility: T.L. Grove Leeman et al. 1990; Conrey et al. 1997; Bacon et al. 1997; 250 Ma rquez et al. 1999), even though the arc in which they 116 49N are found may have no obvious connection to intraplate- Mt. Baker ° W or plume-related tectonic settings or processes. Glacier Peak Here we report age and chemical results from young N lavas of two small basaltic ®elds in the central Wash- 0 km 200 Mt. Rainier ington Cascades that clearly exemplify the CAB-OIB Mt. St. Goat Rocks compositional distinction. We show that this distinction Helens Mt. Adams 46N Simcoe cannot be explained simply by dierent amounts of Boring Indian Heaven ¯uid-rich subduction component (SC) from the sub- Lavas Mt. Hood ducting slab in each mantle source. However, the dis- Lahars of Mt. Rainier Basalt of Three Sisters tinct and divergent CAB-OIB array is easily modeled as and Dalles Lakes ¯ux-melting of the mantle, involving a correlation be- Kent Mt. Rainier basaltic andesite tween the amount of SC added to the mantle source and Mt. Rainier andesite the degree of melting of the source. One important im- and dacite plication of this modeling is that OIB-like arc basalts can Tertiary intrusive be produced by the same melting process and from the rocks 47°15' same mantle source as CAB melts, simply by relatively Puyallup Enumclaw low-degrees of melting of a mantle region that has had a comparatively small amount of slab component added to it. Thus, OIB-like trace element compositions do not Orting Dalles necessarily indicate the presence of plume material or Three Sisters Lakes ocean-island-like melting processes under arcs. 47°00' Samples and methods The Three Sisters (TS) and Dalles Lakes (DL) basalts were erupted from relatively small and presumably short-lived vents about 30 km north of Mt. Rainier Mt. Rainier (Fig. 1). Our mapping, as well as earlier work (Fischer 46°45' 1970; Hartman 1973; Hammond 1980), indicates that 0510 15 20 the TS and DL basalts were erupted subaerially through km after Luedke & Smith (1982) older andesitic porphyry and volcaniclastic deposits. 122°15' 122°00' 121°45' 121°30' Each vent produced approximately 4±10 lava ¯ows with a similar phenocryst content of olivine, and rare pla- Fig. 1 Generalized geologic map of the central Washington Cascades near Mt. Rainier (after Luedke and Smith 1982; gioclase and/or clinopyroxene. The TS and DL basalts Hammond 1998) showing the locations and approximate areal are the only known volcanic rocks of the High Cascades distribution of the Three Sisters and Dalles Lakes basalts. Shown in (late Cenozoic to recent) between Mt. Rainier and the inset are the locations of other Washington and northern Glacier Peak (Hammond 1980; Luedke and Smith 1982), Oregon volcanoes (triangles) and selected volcanic ®elds discussed in this paper (squares). The Three Sisters and Dalles Lakes basalts a distance of nearly 150 km. Their apparently primitive were originally described by Fischer (1970) and Hartman (1973), compositions provide insights into primary magma who named them the Basalts of Canyon Creek and Dalles Ridge, compositions and mantle-melting processes in this part respectively. For stratigraphic clarity (because of the abundance of of the Cascade arc. ``Canyon Creek'' names in the Cascades, and because we reserve the name ``Dalles Ridge'' for one of the thick, older volcanic units We collected and analyzed 7 TS and 11 DL samples that composes most of the ridges in the area) we suggest the new for major- and trace-element compositions (Table 1). names Analyses were performed at Washington State Univer- sity and CHEMEX Labs (Vancouver, B.C.) using standard XRF methods for major elements and ICP-MS than the Three Sisters basalt (Table 1). Single-step fusion for trace elements. Three (including two ¯ows and one ages for two samples (365 50 and 385 71 ka) and dike) TS samples and one DL ¯ow sample were dated by plateau and isochron ages of one sample (398 15 and whole-rock, single-step, total-fusion 40Ar/39Ar methods, 377 42 ka, respectively) indicate an age of about 370± and one TS sample was also dated by incremental 400 ka for the Three Sisters (Table 1). The single-step heating 40Ar/39Ar methods, at Oregon State University. fusion procedure of 96PRTS6 produced a very low ra- diogenic Ar yield and large analytical uncertainty that precludes con®dent age assignment, but it is concordant Results with the results of the other samples. High MgO (8±10 wt%), Mg#(60±70), and compatible 40Ar/39Ar ages indicate that the Dalles Lakes basalt was trace element contents indicate that the DL and TS lavas erupted at 2.2 0.05 (1r) Ma, and is signi®cantly older are primitive basalts that have experienced little 251 Table 1 40Ar/39Ar ages (Ma) of samples of the Dalles Lakes 96PRDR2 96PRTS1 96PRTS6 96PRTS11 and Three Sisters basalts, cen- tral Washington Cascades. Single-step, total fusion age 2.18 0.05 0.385 0.071 0.271 0.174 0.365 0.050 are 1r Radiogenic Ar 89.1% 38.3% 1.30% 74.1% Plateau (2 steps, 80% of gas) 0.398 0.015 Isochron (5 steps) 0.377 0.042 fractional crystallization (Tables 2, 3). In many other (>52.5%) TS samples show slightly dierent trace respects, however, major- and trace-element composi- element trends than the rest of the TS suite, with higher tions of DL and TS lavas are distinct: DL lavas are Ni and incompatible element concentrations. DL lavas alkalic basalt whereas TS lavas are subalkalic basalts also show interesting intrasuite trends, particularly and basaltic andesites (Fig. 2). DL lavas also have high correlations between Ni and incompatible elements such TiO2 and Fe2O3, and low SiO2 relative to the TS lavas as Ba, La, and Sr (Tables 2, 3).